Plant Water Relations: Uptake and Transport (TTPB27) Teaching Guide

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1 Plant Water Relations: Uptake and Transport (TTPB27) Teaching Guide Overview Plants, like all living things, are mostly water. Water is the matrix of life, and its availability determines the distribution and productivity of plants on earth. Vascular plants evolved structures that enable them to transport water long distances with little input of energy, but the hollow tracheary elements are just one of many adaptations that enable plants to cope with a very dry atmosphere. This lecture examines the physical laws that govern water uptake and transport, the biological properties of cells and plant tissues that facilitate it, and the strategies that enable plants to survive in diverse environments. Learning objectives By the end of this lecture the student should be able to: Evaluate the historical context and significance of the studies conducted by Stephen Hales and published in the book Vegetable Staticks Summarize and define each component of the water potential equation Diagram the flow of water through the Soil Plant Atmosphere Continuum, and indicate where the water is under positive or negative pressure Evaluate the significance tracheary elements in plant evolution and map their presence onto a phylogenetic tree Study / exam questions (understanding and comprehension) True or false: Compared to molecules of similar molecular weight, water has an unusually high boiling point. Imagine two adjoining cells, the left one with a water potential of -0.5 MPa and the right one with a water potential of -0.2 MPa. Will the water move from Left to Right, or Right to Left? What is the source of energy that drives water movement through the plant body? The flow of water from soil to root is affected by both the water potential difference between soil and root and the conductance of the root. Describe a condition that would change the water potential gradient across the surface of the root. Describe a condition that would change the hydraulic conductance of the root system. What are aquaporins? In terms of Fick s Law, which parameter do they affect? Describe three ways that aquaporin activity is regulated. How does the Casparian strip affect water movement into the plant? Identify and describe the three pathways through which water can move from the soil to the stele of the root. Xylem is described as a vulnerable pipeline : vulnerable to what? What are pit membranes, what are they composed of, and how do their structural properties affect cavitation resistance? Order these plants according to the water potential at which they experience 50% loss of conductivity, from least negative to most negative: o Sugar maple o Mangrove o Cottonwood o Ceonothus Which are usually wider, tracheids or vessels? How does increased xylem diameter affect flow and sensitivity to cavitation?

2 What is one strategy that a plant with very wide vessels (e.g., grapevine) can use to avoid freezing-induced embolism formation? The high vein density of angiosperm leaves enables them to have a high rate of photosynthesis. Explain the connection between vein density and photosynthesis in terms of hydraulic conductance. Describe two ways that leaves can act as disposable fuses to protect the integrity of the water column in large braches. What are stomatal crypts and how do they influence transpiration: by changing driving force or conductance? You can check your understanding of the uptake and transport of water through this excellent interactive tutorial (At the time of publication, this was a functioning link hopefully it still is.) Discussion questions (engagement and connections) Hales Vegetable Staticks is available online at the Biodiversity Heritage Library, scanned from the Peter H, Raven Library at the Missouri Botanical Garden ( Skim through the book, and examine the drawings that illustrate the experiments. How easy it is to read the text? Can you follow the descriptions of the experiments and their interpretations? Pick one of Hales experiments and rewrite it, using your own words, and interpret the results using our contemporary understanding of the movement of water in plants. Fick s Law, Poiseuille s Law and Ohm s Law are all variations of the same general equation. Compare and contrast the three equations. Can you identify situation in which each would be most appropriate to understand the system? What happens to a plant cell that is placed into a solution of pure water, with a water potential of 0. How do the cell s water potential, osmotic potential and pressure potential change? Why do plants have so many genes encoding aquaporins? Propose three different reasons to explain the large number of genes, and describe an experiment to test each possibility. Investigate the literature to determine if any or all of your proposed explanations are supported by known evidence. Describe three ways that roots can respond to water deficit. Indicate whether each is a rapid or slow response. Compare and contrast the anatomical properties of tracheids and vessels. In what habitats would you expect to find plants with an abundance of tracheids versus vessels, and why? In your own words, explain and diagram the flow of sap in a maple tree during the springtime. What time of day is the best time to collect sap, and why? Why might rising global temperatures interfere with sap collection? See for example Stevens, C.L., and Russell, L. Eggert. (1945). Observations on the causes of the flow of sap in red maple. Plant Physiol. 20: , Tyree, M.T. (1983). Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms. Plant Physiol. 73: , and Cirelli, D., Jagels, R. and Tyree, M.T. (2008). Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch. Tree Physiol. 28:

3 What environmental factors have influenced leaf morphology? What is the adaptive significance of these factors? In what ways do the leaves of angiosperms differ from those of other plants? What are some of the strategies used by leaves of nonangiosperms to increase hydraulic conductance and safety? See for example Brodribb, T.J., Feild, T.S. and Jordan, G.J. (2007). Leaf maximum photosynthetic rate and venation are linked by hydraulics. Plant Physiol. 144: We still do not fully understand how drought causes death. Barigah et al (2013) examined tree responses to water stress, cavitation and death. Read this paper and answer the following questions. Which tree, poplar or beech, is the most resistant to drought? How many weeks after drought began elapsed before the plants showed a loss of conductance? How many weeks before the trees started to die? Of the parameters measured, which was the most similar between the two types of plants? Beech and poplar are both angiosperms would you expect to see similar results for conifers? Why or why not? [Barigah, T.S., Charrier, O., Douris, M., Bonhomme, M., Herbette, S., Améglio, T., Fichot, R., Brignolas, F. and Cochard, H. (2013). Water stress-induced xylem hydraulic failure is a causal factor of tree mortality in beech and poplar. Ann. Bot. 112: ( Lianas are woody vines, usually found in the tropics, that do not support themselves but instead climb up trees; they can be described as structural parasites. They often have interesting hydraulic properties including very wide vessel elements (like grapevines). Lianas seem to be increasing in abundance in many tropical forests, and there is considerable interest in understanding the relationship between lianas and climate change. Starting with the review article by Schnitzer and Bongers (2011), investigate the hydraulic properties of tropical lianas. Do you think that an increasing incidence of drought in tropical regions will preferentially benefit lianas or their hosts? Some references to get you started: Schnitzer, S.A. and Bongers, F. (2011). Increasing liana abundance and biomass in tropical forests: Emerging patterns and putative mechanisms. Ecol. Lett. 14: Johnson, D.M., Domec, J.-C., Woodruff, D.R., McCulloh, K.A. and Meinzer, F.C. (2013). Contrasting hydraulic strategies in two tropical lianas and their host trees. Am. J. Bot. 100: Sande, M., Poorter, L., Schnitzer, S. and Markesteijn, L. (2013). Are lianas more droughttolerant than trees? A test for the role of hydraulic architecture and other stem and leaf traits. Oecologia. 172: Zhu, S.-D. and Cao, K.-F. (2009). Hydraulic properties and photosynthetic rates in cooccurring lianas and trees in a seasonal tropical rainforest in southwestern China. Plant Ecol. 204: Select two methods used for the study of plant water relations to compare and contrast. What does each method measure? What equipment is needed, what artifacts can arise, and what controls are necessary? See Nobel, P.S. (2009). Physicochemical and Environmental Plant Physiology, 4th ed. (Oxford, UK: Academic Press) for quantitative, challenging questions for more advanced students.

4 Lecture synopsis INTRODUCTION (1-4) This is the first part of two Teaching Tools in Plant Biology that examine plant-water relations. Part I examines how plants take up water from their environments and transport it through their bodies, and Part 2 looks at adaptations to water limitation. Water is the matrix of life, and its availability largely determines the distribution and productivity of plants on earth. To understand how water moves in plants, it is important to first understand the properties that make water indispensable for life. Vascular plants, the focus of this lesson, have evolved complex mechanisms by which to absorb, transport and retain water; these include roots, xylem and the water-conducting tracheary elements, and regulated pores (stomata). The structure and function of these features dictate how efficiently and safely plants move water. BRIEF HISTORY OF THE STUDY OF PLANT-WATER RELATIONS (5-9) Following the discovery by William Harvey that blood circulates in mammals, many scientists tried to understand how fluids move in plants. Stephen Hales carried out many insightful studies and correctly concluded that the movement is not usually driven by root pressure, and he ruled out circulation as a significant form of water movement in plants (based on the volume of water transpiring). In the 1890s, several scientists, notably Böhm, Dixon and Joly, converged on the idea that the driving force for the movement of water was the evaporation of water at the leaf surface. WATER UPTAKE AND TRANSPORT ARE GOVERNED BY PHYSICAL LAWS (10 29) Water flows into and through plants as a continuous column of water. Ultimately, the energy required to move this column of water comes from sunlight, which drives the evaporation of water at the leaf surface. As water evaporates from the top of the water column, it exerts tension on the column that draws more water in at the bottom of the column. Special properties of water (11) Water is unusually cohesive, and the molecules stick to each other, so that a column of water can withstand significant tension without breaking. This property allows water to be pulled under a negative pressure. Factors that govern the movement of water Diffusion (12 14) Gaseous and liquid matter tends to become uniformly distributed; this phenomenon can be described by Fick s Law of Diffusion: Ficks s Law:. The rate of movement of a substance (dm / dx) is determined by its concentration gradient (dc / dx), the area through which it is diffusing (A), and the properties of the substance and the matrix through which it is diffusing (defined as the diffusion coefficient, D). Osmosis and osmotic potential (15 16) Osmosis is a special type of diffusion that applies only to water moving across a semipermeable barrier, such as the plasma membrane of a cell. The osmotic potential of pure water is defined as 0 MPa, and the addition of any solute lowers the osmotic potential to negative values. The more dissolved solutes, the more negative the osmotic potential and the greater the tendency for water to move into the solution.

5 Pressure potential (17 18) Water flows from high pressure to low pressure. Most plant cells are under a positive pressure, on the order of 0.5 to 1.5 MPa [more than the pressure of a car tire (which is about 0.2 MPa) and many-fold higher than those experienced by animal cells; for example, human arterial blood pressure is less than 0.02 MPa]). By contrast the water inside the conducting tissues of the xylem is under a negative pressure (tension). Water potential (19 20) Water potential provides a convenient quantification of the energy state of water, differences in which can drive water movement, and it is the sum of osmotic, pressure and gravitational potentials. The standard notation to express this is the water potential equation, The rule governing the movement of water across membranes is that water moves from higher to lower water potential. When no membranes are transversed, for example in the column of water in the xylem or phloem, water s movement is driven by the pressure gradient. Water moves in plants by diffusion and by bulk flow (21 24) The movement of water by diffusion is a relatively slow process. For example, a water molecule can diffuse across a 50 μm cell in 0.6 s, but it would take 8 years to diffuse 1 m. Thus, diffusion alone is insufficient to support the needs of the typical plant for water. Instead, vascular plants rely on bulk flow of water. Bulk flow is a much faster form of water movement, but it also requires an energy source to generate the required pressure gradient. Ohm s Law can model the flow of water through a plant (25-29) Although Ohm s Law is associated with the flow of electricity, it is also a simple and effective model for which to understand the flow of water. It states that flow is equal to the driving force (water potential difference) divided by the resistance to the flow. Higher resistance means lower flow. Conductance is the reciprocal of resistance, so Ohm s law can also state that flow is equal to the driving force times conductance; higher conductance means higher flow. Both conductance and resistance are regularly used in plant physiology. Although whole-plant resistance and conductance can be considered, it is more useful to consider these values separately for separate segments of the plant, as we do here. Conductance can be measured separately for roots, stems, leaves, stomata etc., and by doing so we can better understand how the flow of water is affected by plant morphology, anatomy and physiology. AQUAPORINS, ESSENTIAL REGULATORS OF WATER MOVEMENT (30 37) Although water can move across a cell membrane, its movement is greatly enhanced by the presence of water channels called aquaporins. Aquaporins are found in all organisms. Their activity is controlled transcriptionally, through phosphorylation and dynamic movement between internal and plasma membranes, and channel gating. Regulated aquaporin activity modulates the conductance of plant tissues in response to many parameters including light and drought. WATER MOVES THROUGH THE SOIL PLANT ATMOSPHERE CONTINUUM (SPAC) (38-106) Water evaporates from the soil into the atmosphere. Water movement through a plant is essentially the same process, but occurs by way of the plant body. The movement of water

6 is down a water potential or pressure potential gradient, and the rate of movement is affected by factors that vary the water potential gradient, and factors that alter the conductance of the plant segments. The most intuitive way to consider the how plant physiology, anatomy and morphology affect water movements is to consider the root, stem and leaf separately. SOIL PROPERTIES AFFECT THE MOVEMENT AND UPTAKE OF WATER (40-42) Most vascular plants draw in water from the soil by way of their roots. Factors that affect soil water potential affect water uptake, and these include soil particle size and charge. The capacity of the soil for water influences plants by establishing a how well the soil can hold onto water between irrigation or rainfall events. For example, sandy soils do not hold water very well, and plants growing in sandy soils have to be able to deal with low soil water potentials. Soil particle size and the presence of organic and biotic matter also affects soil water potential and water-holding capacity. SOIL PLANT ATMOSPHERE CONTINUUM 1. WATER UPTAKE IN ROOTS: FROM SOIL TO STELE (43 55) Root architecture affects the volume and depth from which water can be extracted (43 47) The amount of surface area of root is a major factor affecting water uptake. Surface area can be increased by growth and by increasing the number of lateral roots as well as root hairs. The position and orientation of growth can facilitate the uptake of water; growing deep is a strategy to reach deep water reserves. Interestingly, when water is limiting, shoot growth slows dramatically, but root growth, and particularly primary root elongation, is maintained. There is some evidence that roots can sense and grow towards water as well (known as hydrotropism). Root conductance affects water flow (48-54) Water flow in the root can be considered as radial (from surface to xylem) and axial (along the long axis, in the xylem). Radial conductance is usually much less than axial conductance. Radial conductance involves movement through cells, and includes apoplastic (through cell walls), symplastic (through cells and plasmodesmata between cells), and transcellular (from cell to cell through aquaporins) pathways. The endodermis and exodermis are cell layers with lignified, hydrophobic barriers embedded in their cell walls that decrease conductance (and form essential barriers to ions and other dissolved solutes). SOIL PLANT ATMOSPHERE CONTINUUM 2: FLOW OF WATER THROUGH THE XYLEM (55-89) Evolution of xylem and lignin (55 60) Bryophytes have some specialized transport cells called hydroids that contribute to water movement, but don t match the efficiency of the tracheary elements (TEs) that define tracheophytes (vascular plants). TEs are hollow, dead cells the conduct water very efficiently. Looking at the fossil record, it is clear that these cells have become wider and their cell walls more elaborate in the more than 400 million years since they first appeared. Ligin is a complex, hydrophobic polymer that strengthens the secondary cell walls of the tracheary elements and provides structural support as well as resistance to tension.

7 Development and differentiation of tracheary elements (61-64) The formation of the xylem starts with the patterning that goes on at the apical meristem, and in developing leaves and lateral roots. The precursors of TEs elongate, lay down a thickened secondary wall, and then undergo programmed cell death. Many plants are able to continuously produce new xylem from the vascular cambium; this allows plants to grow in girth but also to replace non-functional, embolized xylem. Structure and function of tracheary elements (65-76) We can describe TEs as tracheids (mostly in gymnosperms, ferns and lycophytes) and vessel elements (mostly angiosperms). These two types of TEs differ in structure and function. Tracheids are fairly narrow, tapering cells. Vessel elements are wider and more cylindrical, and often open at the ends. They stack together into vessels, which can be very long. TEs also differ in the structure of the bordered pits that connect adjacent cells. The vulnerable pipeline (68-76) Because water in the xylem is under tension, any air bubbles that form in it tend to expand. Air can enter the xylem through the pit membranes, a condition known as cavitation. A conduit filled with an expanded air bubble is referred to as an embolism, and embolized conduits cannot carry water. As xylem cavitates and embolizes, it loses efficiency. Tracheids are often more resistant to embolism and cavitation than vessel elements. There is a tradeoff between efficiency and safety in xylem, which is adapted to the plant s environment. A FEW EXAMPLES TO ILLUSTRATE WATER MOVEMENT IN XYLEM Mangroves and reverse osmosis of seawater (77 79) Per Scholander recognized that mangroves face an additional challenge as compared to other plants, because they have to pull in fresh water from salt water, through a reverse osmosis process. The endodermis helps to filter out the salt, but because of the very low water potential of salt water, the xylem tension must also be very low to draw in water. Scholander measured xylem tension and found it to be quite low as compared to plants growing in a fresh water environment. The water potential of cells in the leaf also are quite low, due to the presence of dissolved solutes including organic molecules such as proline. Does xylem conductance limit the height of trees? (80) The water potential of a column of water gets more negative with height, because of the gravitational pull on it. The question posed is still being debated (see references). Xylem under pressure: The roles of root and stem pressure on water movement (81 99) Refilling of grapevines by root pressure (81 82) Grapevines have very wide vessels, which makes them particularly vulnerable to freezing-induced embolism. The vessels empty over the winter and must refill in the spring, which involves pressure from the roots. This is similar to the phenomenon of guttation in which plants exude water that sometimes happens in the morning. Stem pressure and flow of maple sap (83 88) The spring flow of maple sap also involves pressurized xylem, but it involves pressure that builds in the shoot, not root. Maple sap flow has provided an interesting case study for plant physiologists for years, and provides an interesting thought experiment for students, although all of the details are still being worked out (see references).

8 SOIL PLANT ATMOSPHERE CONTINUUM 3: FROM LEAF TO AIR (90-106) Leaf vein anatomy and leaf conductance (90 98) As described earlier, xylem conductance is usually greater than out-of-xylem conductance (through apoplastic, symplastic and transcellular pathways). The vein pattern in leaves affects their hydraulic efficiency, and angiosperms, with their very high rates of photosynthesis, tend to have denser veins than other plants, although some gymnosperms have specialized, highly conducting cells outside the xylem. Interestingly, the xylem in leaves and small twigs can be more vulnerable to embolism than the xylem in larger branches and stems, and this is thought to cause these tissues to act as disposable fuses that protect the stems (that represent a greater energy investment). Some conifers also have tracheids that can reversible collapse and protect the stem xylem from very low pressures and risks of cavitation. After leaving the xylem, water passes through bundle sheath cells that may have some of the same selectivity functions as the endodermis. Aquaporin activity in the leaf cells contributes to the regulation of the leaf s hydraulic conductance. Stomatal conductance (99 105) The flow of water vapor through stomata is determined by stomatal conductance and by the water potential gradient across the pore. The guard cells that regulate stomatal pore aperture are some of the most interesting and dynamic plant cells (they are covered in depth in a separate Teaching Tool). The movement of ions (and water) into and out of the guard cells controls stomatal aperture and so transpiration, and the activity of many ion channels in the plasma membrane and tonoplast membrane are tightly regulated. DROUGHT, HYDRAULIC FAILURE, AND WHAT IT ALL MEANS ( ) Plants must balance their need to assimilate CO 2 and their need to avoid drying out, and plants vary in their hydraulic efficiency and safety depending on the niche they occupy and competition with species occupying similar niches. Most plants can tolerate daily and seasonal fluctuations in water availability, but prolonged drought is fatal for most plants. There is a disturbing increase in drought-associated tree mortality that is thought to reflect the changing rainfall patterns occurring across the globe. In managed or irrigated environments, drought effects can be ameliorated by irrigation, but this is not feasible for most of the world s forests. As summarized in a recent report from the USDA, Although some regions will be affected more than others, these disturbances are likely to change the structure and function of ecosystems across millions of acres over a short period of time with detrimental effects on forest resources. (Vose et al., (2012) Effects of climatic variability and change on forest ecosystems: a comprehensive science synthesis for the U.S. forest sector. USDA. SUMMARY (111) From dynamic root growth patterns to regulated aquaporin channels, and with an amazing integration of xylem structure and function and sophisticated leaf anatomy, the mechanisms by which plants take up and transport water are spectacular METHODS FOR MEASURING WATER MOVEMENT IN PLANTS ( ) Without getting too deeply into the controversies, we introduce a few of the methods commonly used, and refer the reader to the recommended readings for more thorough discussions. Methods are introduced for measuring transpiration, xylem tension, flow, conductance, percentage loss of conductivity, and embolisms.

9 Slide concepts Slides Table of contents concepts 1 Plant-water relations (1) - Uptake and transport 2 Water is a major factor in plant distribution 3 Plants have evolved several strategies to manage water 4 Outline: Plant-water relations Uptake and transport 5 Brief history of the study of plant-water relations 6 Malpighi and Grew observed conducting materials ( s) 7 Hales investigated water movement in Vegetable Staticks (1727) 8 Hales showed that water movement requires leaves exposed to air 9 Dixon and Joly: The Cohesion-Tension Theory (1890s) 10 Water uptake and transport are governed by physical laws 11 Water has many unusual and important properties 12 Fick s law describes the movement of water by diffusion 13 Diffusion rate is affected by area, distance, and gradient 14 Diffusion rate is affected by the properties of material and solvent 15 Water moves down osmotic gradients 16 Osmotic forces drive the movement of water into and out of cells 17 Pressure can be positive or negative 18 Turgor pressure supports plant cells and tissues 19 The water potential equation incorporates osmotic and pressure potentials 20 Water moves towards lower water potential 21 Water moves by diffusion and bulk flow 22 Diffusion rate is affected by the properties of material and solvent 23 Long-distance water movement occurs by bulk flow 24 Bulk flow through a tube is affected by the diameter of the tube 25 Ohm s law can model water flow, whether diffusion or bulk flow 26 Flow rate is the product of driving force and hydraulic conductance 27 Resistance and conductance are reciprocals & both affect flow 28 Water flow is governed by the conductance of 3 plant segments 29 Summary: Water movement is governed by physical laws 30 Aquaporins are regulated membrane-bound water channels 31 Arabidopsis has 35 aquaporin genes in four families 32 Protoplast swelling assay of cell permeability and aquaporin s roles 33 Aquaporin activity is regulated at many different levels, including transcription 34 Aquaporin activities are also regulated post-transcriptionally 35 Aquaporin channels are gated and can be open or closed 36 Aquaporins facilitate rapid plant movements 37 Aquaporin expression and activity affect hydraulic conductance 38 Water moves through Soil Plant Atmosphere Continuum (SPAC) 39 The driving force for water flow is evaporation powered by solar energy 40 Soil properties affect the movement of water 41 Molecular interactions between water and soil affect water uptake 42 The type of soil particle (shape, size) affects soil interactions with water

10 43 Soil Plant Atmosphere Continuum 1. Water uptake in roots: From soil to stele 44 Under water deficit, root growth is maintained relative to shoot growth 45 Water flow into roots is affected by root architecture and conductance 46 The distribution of root growth is affected by water availability 47 Root growth can respond to water gradients (hydrotropism) 48 Root conductance is determined by radial and axial conductance 49 Water likely moves from soil to stele through multiple paths 50 Conductance of cell walls and membranes affects water uptake 51 The Casparian strip forces water to cross a plasma membrane 52 The root endodermis acts as a filter for incoming water 53 Robert Caspary (1865) recognized the significance of this barrier 54 Summary: Movement of water into and through roots 55 Soil Plant Atmosphere Continuum 2: Flow of water through the xylem 56 Transport in bryophytes some specialized transport cells 57 Hollow conducting tissues allow greater height and CO 2 assimilation 58 Fossil record: Xylem conduits became wider and more complex 59 The secondary walls are reinforced by lignin, a water-impermeable polymer 60 Lignified xylem provides structural support for vascular plants 61 Development and differentiation of tracheary elements 62 Tracheary elements have been described as functional corpses 63 Most seed plants (and a few others) can produce secondary xylem 64 Secondary thickening produces new xylem and lets plants increase in girth 65 How does xylem structure affect its function? 66 Xylem anatomy: Hollow, thickened conducting cells 67 Tracheids and vessels are quite different in size 68 Xylem is a vulnerable pipeline, prone to embolism 69 Cavitation describes a condition wherein an air bubble moves into a vessel 70 Embolisms can spread between vessels 71 Pit membranes permit water flow and limit embolism spread 72 Some pits are more embolism-resistant than others 73 Vulnerability curves characterize the sensitivity of a plant to embolism 74 Many plants experience a seasonal loss and recovery of conductivity 75 Vulnerability to embolism is ecologically relevant 76 There is a correlation between habitat and cavitation resistance 77 The environment affects xylem tension: Mangroves 78 Scholander showed that tension in the xylem drives water uptake 79 Tension in the xylem is balanced by solute accumulation in the leaves 80 Does xylem conductance limit the height of trees? 81 Xylem under pressure: Root pressure and grapevine refilling 82 Hollow vessels fill with water in the spring due to root pressure 83 Stem pressure and the flow of sap in maple trees (Acer saccharum) 84 Clues to the source of maple sap 85 As long as water is provided, sap flow does not require roots or crown 86 Sap flow requires temperatures alternating above and below freezing sap flow model: Freezing compresses vapor in xylem fibers

11 model: Contribution of sugars in minimally pitted vessels 89 Summary: Water flow through xylem, a low-resistance pathway 90 Soil Plant Atmosphere Continuum 3: From leaf to air 91 Leaf conductance (K leaf ): Xylem and out-of-xylem conductance 92 Leaf xylem anatomy affects leaf hydraulic properties 93 The distance from the end of the conducting cells to the air matters 94 Water conducting sclereids shorten D m and increase K leaf 95 Leaf and small branch xylem can act as disposable fuses 96 In conifer tracheids, xylem collapse in the needles protects stem xylem 97 K ox : Water leaving the xylem passes through bundle sheath cells 98 Aquaporins contribute to radial conductance in the leaf 99 Guard cells are the gatekeepers of transpiration 100 Guard cells are the portals through which CO 2 enters and H 2 O exits 101 Guard cell movements are controlled by ion currents 102 Exiting ions draw water out of the cell 103 The guard cells lose turgor pressure and relax, closing the pore 104 What regulates stomatal aperture? 105 Stomatal phylogeny, anatomy and morphology affect function 106 From leaf to air: Summary 107 Drought, hydraulic failure and what it all means 108 A disturbing and widespread increase in tree mortality is occurring 109 This trend is taking place world-wide 110 Drought has pleiotropic effects and increases vulnerability to pests 111 Water Uptake and Transport: Summary 112 Methods used in plant-water relations research 113 Pressure to balance xylem tension can be measured in pressure bomb 114 A pressure probe can also measure xylem tension, but it is difficult 115 Flow rate can be measured by the rate at which heat moves 116 Conductance can be measured as flow through a segment 117 Protoplast swelling assay measures cell membrane water conductance 118 Percentage loss of conductivity can be measured by lowering ψ w 119 Xylem ψ w can be lowered by drying or centrifugation 120 Maximal conductance can be measured after refilling embolisms 121 Embolism visualization by Cryo-SEM and dye uptake studies 122 3D visualizations using high resolution computed tomography